|Publication number||US7573953 B2|
|Application number||US 11/205,561|
|Publication date||Aug 11, 2009|
|Filing date||Aug 17, 2005|
|Priority date||Aug 18, 2004|
|Also published as||EP1628451A2, EP1628451A3, US20060039507|
|Publication number||11205561, 205561, US 7573953 B2, US 7573953B2, US-B2-7573953, US7573953 B2, US7573953B2|
|Inventors||Dong Geun Lee|
|Original Assignee||Lg Electronics Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (8), Non-Patent Citations (3), Referenced by (2), Classifications (7), Legal Events (3)|
|External Links: USPTO, USPTO Assignment, Espacenet|
Pursuant to 35 U.S.C. § 119(a), this application claims the benefit of earlier filing date and right of priority to Korean Patent Application No. 10-2004-0065129, filed on Aug. 18, 2004, the content of which is hereby incorporated by reference herein in its entirety.
1. Field of the Invention
The present invention relates to a digital receiver, and more particularly, to a method and apparatus for estimating an SFO (Sampling Frequency Offset) applicable to the digital receiver, and an apparatus for compensating for a sampling frequency using the estimated SFO.
2. Discussion of the Related Art
Generally, a DVB-T (Digital Video Broadcasting-Terrestrial) system acting as the European transmission standard for a terrestrial digital TV has generally selected an OFDM (Orthogonal Frequency Division Multiplexing) transmission scheme. It is well known in the art that the OFDM transmission scheme has very strong resistance to channel distortion caused by multiple paths (also called a multi-path) in a wireless broadband broadcast system.
On the other hand, the OFDM transmission scheme has very weak resistance to synchronization. Therefore, if accurate synchronization is not established between a transmitter and a receiver, distortion of a reception signal occurs. In order to solve the above-mentioned problem, many developers have conducted intensive research into the improved OFDM transmission scheme.
Particularly, if the receiver does not correctly perform sampling synchronization, an ISI (inter Symbol Interference) and a constellation rotation may occur in a reception signal, such that the receiver cannot demodulate the reception signal.
In order to solve the above-mentioned problem, there has been newly proposed a method for estimating an SFO (Sampling Frequency Offset) using a CP (Continual Pilot) shown in
For example, in the case of the 2 k mode, a total of 1705 data subcarriers are present in one OFDM symbol interval. A pilot is located at each of subcarrier positions, for example, 0-th, 48-th, and 54-th subcarrier positions, etc. In this case, the pilot is positioned at the same subcarrier positions as the above subcarrier positions in the next OFDM symbol, such that the pilot will be referred to as a Continual Pilot (CP).
A method for calculating the SFO using the above-mentioned CP information shown in
The above-mentioned conventional SFO estimation method will hereinafter be described with reference to
The Zl,k signal is converted into another signal of Zl-1,k via a delay 10. The Zl-1,k signal is converted into a conjugate root signal of Z*l-1,k via a conjugate calculator 20.
Correlation between the Zl,k signal and the Z*l-1,k signal is performed by a multiplier 30, such that the multiplier 30 outputs a phase information signal of xl,k. By the following equation 1 performed by a phase estimator 40, the phase information signal of xl,k acquires a total of 45 phase data units in the case of the 2 k mode, and acquires a total of 177 phase data units in the case of the 8 k mode.
An SFO calculator 50 calculates a slope between phase data units using the phase data generated from the phase estimator 40, and calculates a mean slope, such that it calculates the SFO value.
The above-mentioned conventional SFO estimation method acquires correlation between two OFDM symbols, acquires a phase on the basis of the acquired correlation, and calculates a change rate of the acquired phase, such that it estimates the SFO value.
However, the above-mentioned SFO estimation method may incur irregular overshoots of the phase due to a deep fading phenomenon in a long ghost environment as shown in
The above-mentioned irregular overshoots do not affect the SFO in an acquisition mode of a sampling frequency, but it greatly affects the SFO in a tracking mode of the sampling frequency as shown in
In this manner, if the jittering range increases due to the overshoots in the tracking mode, the increased jittering range has a negative influence upon a method for compensating for a sampling frequency by estimating a correct SFO. Furthermore, assuming that an SFO of more than 400 ppm occurs in the 2 k mode of the long ghost environment or an SFO of more than 100 ppm occurs in the 8 k mode of the same long ghost environment, the conventional SFO estimation method has difficulty in correctly estimating the above-mentioned SFO values.
Accordingly, the present invention is directed to a method and apparatus for estimating an SFO in a digital receiver, and a method and apparatus for compensating for a sampling frequency using the estimated SFO.
An object of the present invention is to provide an SFO estimation method for efficiently estimating an SFO, and correctly estimating the SFO although the estimated SFO is transitioned to 400 ppm in a 2 k mode or 100 ppm in an 8 k mode.
Another object of the present invention is to provide an apparatus for compensating for a sampling frequency using the estimated SFO.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a method for estimating an SFO (Sampling Frequency Offset) in a digital receiver to perform sampling synchronization, comprises the steps of (a) receiving an estimated phase, and comparing a magnitude of the estimated phase with that of a predetermined standard deviation; (b) if the magnitude of the estimated phase is greater than that of the predetermined standard deviation, compulsorily setting the estimated phase to zero, and storing the estimated phase of zero; and (c) calculating an SFO value using the stored phase.
Preferably, the estimated phase is estimated using CP (Continual Pilot) information contained in a transmitted OFDM (Orthogonal Frequency Division Multiplexing) symbol.
Preferably, the step (a) for comparing the estimated phase with the predetermined standard deviation is executed when a current frequency estimation mode is equal to a tracking mode. Preferably, if the frequency estimation mode is not equal to the tracking mode, the estimated phase is stored in a buffer, the SFO is calculated using the stored estimated phase.
Preferably, the method further comprises the steps of: receiving the calculated SFO value, and comparing the received SFO value with an input SFO threshold value; if the calculated SFO value is higher than the SFO threshold value, re-calculating the SFO value using only a half pilot phase information.
Preferably, the half the pilot phase information does not include both-end information of an inverted pilot phase.
In another aspect of the present invention, there is provided an apparatus for estimating a sampling frequency offset (SFO) in a digital receiver to perform sampling synchronization, comprising a phase estimator for receiving phase information xl,k, and generating an estimated phase; and an SFO calculator for receiving the estimated phase, comparing a magnitude of the estimated phase with that of a predetermined standard deviation, compulsorily setting the estimated phase to zero if the magnitude of the estimated phase is greater than that of the predetermined standard deviation, and calculating an SFO value.
In still another aspect of the present invention, there is provided a method for compensating for a sampling frequency in a digital receiver, comprising the steps of (a) extracting CP (Continual Pilot) information from a transmitted OFDM symbol; (b) estimating a phase using the extracted CP information, compulsorily setting the estimated phase to zero if a magnitude of the estimated phase is greater than that of a predetermined standard deviation, calculating a SFO (Sampling Frequency Offset) value, and re-calculating the SFO value using only a half a pilot phase if the calculated SFO value is higher than a predetermined threshold value; (c) accumulatively compensating for the calculated SFO value; and (d) outputting a signal for controlling a sampling frequency using the accumulatively-compensated SFO value.
Therefore, the present invention performs zero-forcing on a phase overshoot caused by a ghost, such that a correct SFO can be estimated in the tracking mode.
It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings:
Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.
Prior to describing the present invention, it should be noted that most terms disclosed in the present invention correspond to general terms well known in the art, but some terms have been selected by the applicant as necessary and will hereinafter be disclosed in the following description of the present invention. Therefore, it is preferable that the terms defined by the applicant be understood on the basis of their meanings in the present invention.
As shown in
If the SFO calculator 50 receives the estimated phase ∠x′l,k
If the frequency estimation mode is not equal to the tracking mode at step S10, the SFO calculator 50 receives the estimated phase ∠x′l,k
If the frequency estimation mode is determined to be the tracking mode at step S10, the SFO calculator 50 compares the estimated phase ∠x′l,k
If the magnitude of the estimated phase ∠x′l,k
The above-mentioned operation is called a zero-focusing operation. In other words, the estimated phase is compulsorily set to zero, and the resultant phase is stored in a buffer of the SFO calculator 50 as denoted by Φl,n=0 at step S40.
If the magnitude of the estimated phase is equal to or less than the magnitude of the standard deviation, the estimated phase is stored in the buffer as denoted by Φl,n=∠x′l,k
Thereafter, a slope between phase data units, i.e., an SFO denoted by a specific character “ζ”, is calculated using the above-mentioned stored phase values at step S60. In other words, the SFO calculator 50 calculates a slope among 45 pilot phases in the 2 k mode, and calculates a slope among 177 pilot phases in the 8 k mode as denoted by the following equation 2:
wherein ζ is the calculated SFO.
In the case of calculating the SFO using the above-mentioned method, a phase overshoot caused by a ghost is zero-forced, such that the SFO can be correctly estimated even in the tracking mode.
If the estimated SFO value is higher than a predetermined value of 400 ppm in the 2 k mode, or is higher than a predetermined value of 100 ppm in the 8 k mode, pilot phase information units of both ends are inverted from each other, such that it is difficult to estimate a correct SFO value. In order to solve the above-mentioned problem, there has been used a method shown in
If the calculated SFO value ζ is higher than the SFO threshold value at step S10, the SFO calculator 50 discards the pilot phase information units of both ends, and re-calculates the SFO value using only a half the pilot phase information at step S20. The above-mentioned calculation process can be represented by the following equation 4:
wherein ζ′ is the re-calculated SFO.
Needless to say, if the calculated SFO value ζ is equal to or less than the SFO threshold value, the SFO calculator 50 uses the calculated SFO value ζ without any change.
As shown in
Particularly, the SFO compensator 600 according to the present invention includes a CP extractor 610 for extracting a CP signal from the frequency-domain signal; a timing error detector 630 for receiving the extracted CP signal, and calculating a timing error value equal to the SFO value ζ using the extraction algorithm of the present invention; a loop filter (L/F) 650 for receiving the SFO value ζ, and accumulatively compensating for the received SFO value ζ, and a numerical controlled oscillator (NCO) 670 for controlling a sampling frequency upon receiving the corrected SFO value ζ from the L/F 650.
Therefore, the resampler 300 performs the sampling process using the resultant SFO value correctly estimated by the above-mentioned inventive method, such that correct sampling synchronization is provided.
In the meantime, the present invention is applicable to a DVB-T receiver and a DVB-H receiver.
As apparent from the above description, a method for estimating an SFO in a digital receiver, and an apparatus for compensating for a sampling frequency using the estimated SFO have the following effects.
Firstly, a phase overshoot caused by a ghost is zero-forced (i.e., the phase overshoot is set to zero), such that a correct SFO can be estimated in the tracking mode.
Secondly, the SFO value can be correctly estimated although the estimated SFO is transitioned to 400 ppm in the 2 k mode or 100 ppm in the 8 k mode.
Thirdly, in the case of employing the correctly-estimated SFO value, the sampling frequency can be effectively compensated in the digital receiver.
It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
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|U.S. Classification||375/326, 375/355|
|Cooperative Classification||H04L27/2688, H04L27/2662|
|European Classification||H04L27/26M5C9C5, H04L27/26M5C5|
|Aug 17, 2005||AS||Assignment|
Owner name: LG ELECTRONICS INC., KOREA, REPUBLIC OF
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:LEE, DONG GEUN;REEL/FRAME:016885/0734
Effective date: 20050817
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